The present invention relates to an improved method whereby a hydrogenation catalyst is conveniently and effectively dispersed in and contacted with a reaction mixture. It relates particularly to an improved method for dispersing such catalysts in heavy petroleum fractions and in liquid hydrocarbon slurries of coal prior to hydrogenation.
The hydrogenation of finely divided coal, residual oil, or other such heavy hydrocarbonaceous substances to mixtures of gaseous and liquid products has been studied for many years. In recent years, the liquefaction of coal in particular has become of more urgent interest because of dwindling petroleum resources. Although coal can be successfully hydrogenated to produce both aqueous and liquid products without the addition of a hydrogenation catalyst, since traces of catalytically active metals are normally present in coal, better yields of the desired products are obtained under more moderate reaction conditions when a metal hydrogenation catalyst is used.
Active catalysts for these processes constitute a known class including the metals or compounds of the metals iron, nickel, cobalt, molybdenum, tungsten, tin, zinc, vanadium, chromium, antimony, and a number of others, alone or in combination. Active metals such as palladium, platinum, and rhenium are also effective but are too expensive for the purpose. These hydrogenation catalysts can be added to the hydrogenation mixture as the finely divided metals or as compounds thereof, either supported or unsupported. In the hydrogenation of coal and heavy petroleum fractions, the predominant stable form for most of these metal catalysts is the sulfide which can be formed during the hydrogenation process from the sulfur naturally present in these fossil hydrocarbonaceous substances or by presulfiding the catalyst.
Two well researched processes use a bed of particulate catalyst, usually a nickel or cobalt molybdate supported on alumina, through which are pumped a mixture of hydrogen and a dispersion of finely divided coal in a liquid hydrocarbon medium or a heavy hydrocarbon fraction at elevated temperature and pressure. The Synthoil process, developed at the U.S. Bureau of Mines for the hydrogenation of coal, employs a stationary bed of pelleted or granular catalyst. The process is outlined by Yavorsky et al. in Chem. Eng. Progess 69(3), 51-2 (1973). The "ebullated bed" or H-Coal process employs a bed of similar but more finely divided catalyst which is maintained in the reactor in a turbulent or boiling state as the reaction mixture is passed through it, thereby maximizing contact with the catalyst particles. This process has been used both for the hydrogenation of coal and the hydrogenation of residual oil. The process and variations of it are described in a series of patents of which Johanson, U.S. Pat. Re No. 25,770; Schuman et al., U.S. Pat. No. 3,321,393; and Wolk et al., U.S. Pat. No. 3,338,820 are representative, see also Oil & Gas J. 74 (35) 52-3 (1976). Both of the above processes are effective for the purpose but have inherent difficulties or disadvantages associated with the use of a bed of catalyst, that is, the necessity for specially designed apparatus, the need to avoid occlusion of the catalyst by feed material, the need to avoid caking or plugging in process equipment by particles of catalyst, the deactivation of the catalyst by components of the feed material, and the problems of loading fresh catalyst in the reactor and the removal of spent catalyst. Loss of catalyst fines in the product oil is another problem in this process.
Other known coal and residual oil hydrogenation processes have added a catalyst directly to the reaction mixture as the finely divided metal or a metal compound, either of which may be converted to a catalytically active form under reaction conditions. Some processes have used the metal hydrogenation catalyst in the form of a water-soluble salt, with or without added water. Schuman, U.S. Pat. No. 3,745,108 employs a liquid medium for a coal slurry which is part or all water containing a salt of the metal catalyst in solution, ammonium heptamolybdate ((NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O) being exemplified. This latter process is effective, but the maintenance of a liquid aqueous phase in the process imposes certain limitations on the process conditions and apparatus. Thus, the process temperature is necessarily rather low with resultant comparatively low conversion of coal and low yield of liquid hydrocarbons. In coal hydrogenation, more effective use of the catalyst has been obtained in some cases by prior impregnation of the coal with a catalyst compound, see Pelipetz, U.S. Pat. No. 2,860,101, and Schuman, cited above, for example.